This project aims to establish the mechanisms of action of PHF6 (plant homeodomain finger 6), a new tumor suppressor gene mutated in 20% of primary T-cell acute lymphoblastic leukemia (T- ALL) samples. Notably, the PHF6 gene is located on chromosome X and PHF6 mutations are almost exclusively found in male patients with T-ALL. In addition PHF6 mutations are characteristically associated with activation of TLX1 and TLX3, two highly related transcription factor oncogenes activated by chromosomal translocations in T-ALL. Moreover, PHF6 has two PHD domains involved in the recognition of epigenetic histone marks, which suggests a role in the epigenetic regulation of gene expression. In addition, our preliminary results demonstrate that PHF6 localizes to the chromatin and interacts with the NuRD complex and Arginine methyl transferases implicated in the writing of this posttranslational modification in histone tails. Our central hypothesis is that PHF6 mutations may contribute to the pathogenesis of T-ALL by disrupting specific epigenetic mechanisms that regulate cell proliferation and survival in T-cell progenitor cells. Furthermore, we propose that the aberrant expression of TLX transcription factor oncogenes cooperates with mutational loss of PHF6 in T-cell transformation and that this constitutes a distinct oncogenic pathway in the pathogenesis of T- ALL. Thus, the goals of this research proposal are to define the molecular and cellular functions of PHF6 and to analyze the tumor suppressor activity of this gene in the context of TLX1 induced T-ALL. Towards this goal we will analyze the role of PHF6 in the recognition of epigenetic marks and the control of gene expression and test the tumor suppressor function of Phf6 in vivo using genetically manipulated animal models.